feat(persistence): add LRU draft cache with quota management (#8518)

## Summary

Adds an LRU (Least Recently Used) cache layer and storage I/O utilities
that handle localStorage quota limits gracefully. When storage is full,
the oldest drafts are automatically evicted to make room for new ones.

## Changes

- **What**: 
- `draftCacheV2.ts` - In-memory LRU cache with configurable max entries
(default 32)
- `storageIO.ts` - Storage read/write with automatic quota management
and eviction
- **Why**: Users experience `QuotaExceededError` when localStorage fills
up with workflow drafts, breaking auto-save functionality

## Review Focus

- LRU eviction logic in `draftCacheV2.ts`
- Quota error handling and recovery in `storageIO.ts`

---
*Part 2 of 4 in the workflow persistence improvements stack*

---------

Co-authored-by: Amp <amp@ampcode.com>

knip.config.ts

@@ -64,7 +64,8 @@ const config: KnipConfig = {
   },
   tags: [
     '-knipIgnoreUnusedButUsedByCustomNodes',
-    '-knipIgnoreUnusedButUsedByVueNodesBranch'
+    '-knipIgnoreUnusedButUsedByVueNodesBranch',
+    '-knipIgnoreUsedByStackedPR'
   ]
 }